Data compression systems are well known. Essentially, data compression systems operate on an original date stream, or file, and exploit the redundancies in the data and/or remove superfluous data to reduce the size of the data to a compressed format for transmission or storage. When it is desired to use the data, it is decompressed to a form in which it may be used normally. There are essentially two forms of data compression system, namely reversible (lossless) and irreversible (lossy) systems.
Reversible compression systems are used when it is necessary that the original data be recovered exactly, and these systems are generally used for data such as executable program files, database records, etc. Reversible compression systems include Huffman coding, arithmetic coding, delta modulation, and LZW compression. Depending upon the amount of redundancy in the data (the entropy of the data) to be compressed, reversible compression systems can typically provide a compression ratio of about 2 or 3 to 1 (expressed as 2:1 or 3:1).
Irreversible compression systems are used when it is not required that the original data be recovered exactly and an acceptable approximation of the data can be employed instead. Unlike reversible compression systems, irreversible compression systems can be designed to provide almost any desired compression ratio, depending only upon the standards to which the recovered approximation of the data is subject.
One common use for irreversible compression systems is image compression, as images generally can undergo irreversible compression and decompression with visually acceptable results. For example, digital still images are often processed with the JPEG (Joint Photographic Expert Group) compression system for storage and/or transmission. Depending upon the intended use for the recovered image, JPEG systems can be set to various desired compression ratios, generally between 2:1 and 40:1, although it should be noted that undesired artifacts of compression (blocking, moire patterning, “denting & bruising”, color quantization etc.) tend to dominate smaller images compressed past 12:1 when using a standard JPEG system.
Video images can also be compressed with irreversible compression systems, and the MPEG (Moving Pictures Expert Group) and MPEG-II compression standards have been proposed as reasonable systems for use in such applications. However, typical undesired artifacts of compression for MPEG include all of the JPEG artifacts plus “glittering” at moving edges and color pulsing. The glittering artifacts are due to pixels with values which are far from a block's mean, shifting the average luminance and/or chrominance of a block from frame to frame as these outlying-valued pixels migrate from block to block due to motion of objects in the scene or motion/zooming of the camera.
Irreversible compression systems trade increased compression ratios for decreased quality of the recovered image i.e; higher compression ratio, poorer approximation of the data. Unfortunately, at the higher compression ratios requested/required by video providers and others, all of the prior art irreversible compression systems known to the present inventor result in recovered images of unacceptable visual quality. For example, a still 256×256 pixel monochrome 8 bit image compressed at a compression ration of more than 12:1 with JPEG systems generally exhibit an unacceptable blockiness, which is an artifact of the discrete cosine transform (DCT) block processing stage of the JPEG system. Similar images compressed by a JPEG system to similar ratios present unacceptable “banding” effects near high contrast boundaries.
Generally, the visible degradations of a recovered image are referred to as compression artifacts and attention has been directed to developing irreversible compression systems whose artifacts are unnoticeable or at least less noticeable to the human visual system, and to developing reversible compression systems with lower entropy resulting from better analytic procedures.